An elevator cab assembly will comprise a passenger cab which is mounted in a frame. The cab assembly moves up and down in the elevator hoistway along guide rails which are mounted on opposite walls of the hoistway. Guidance systems, such as rollers, guides, or the like are mounted on the cab frame and engage the guide rails to stabilize the cab assembly as it moves up and down in the hoistway. The guide rails are typically T-shaped and include a blade part which extends toward the cab frame and is engaged by the cab assembly guidance systems. When guide rollers are used as the guidance system, each guidance assembly will include three cooperating rollers arranged in a cluster wherein: an opposed pair of the rollers engages opposite sides of the guide rail blade to provide front and back stability; and a third roller engages the end face of the blade to provide side-to-side stability. There is a roller cluster mounted at each corner of the cab frame. The two upper corner roller clusters are mounted on base plates which project beyond the sides of the frame, and which have slots formed therein to receive the guide rail blades. A coverplate is also mounted over these two clusters to protect the rollers from hoistway debris. The cover plates have slots for receiving the guide rail blades. The rollers are each mounted on lever arms on their respective base plates which are spring biased so as to press the rollers resiliently against the guide rail blades. U.S. Pat. Nos. 3,087,583 to W. H. Bruns and 3,099,334 to B. W. Tucker, Jr. disclose typical prior art elevator cab assembly guidance systems of the roller cluster type described above. As disclosed in U.S. Pat. No. 3,099,334, there are adjustable stops provided on the roller pivot arms or bell cranks which limit the extent to which the rollers can pivot away from the guide rail blades so as to prevent the latter from touching the slots in the base plate, and also in the cover plate for the upper roller clusters. These prior art roller guide systems provide an acceptable quality ride so long as the cab is relatively evenly loaded with passengers, i.e., so long as the cumulative weight of the passengers is evenly distributed in the cab. So long as there is even passenger loading, the roller lever arms will not be pivoted to the extent necessary to ride on the stops for any length of time. In the event, however, that the cab becomes unevenly loaded, as is often the case, the cab assembly's center of gravity will shift, and the cab assembly will tend to tilt or cant to one side, or backward or forward in the hoistway. The reason this occurs is because the cab is suspended in the hoistway on cables which are generally disposed close to an imaginary vertical line which passes through the center of gravity of the cab assembly when empty of passengers. When the cab assembly is unevenly loaded, the side thrust forces imposed on the guide rollers will not be equal, whereby some of the rollers will be subjected to abnormally high forces by the guide rails. These high forces can cause the roller pivot links to pivot against the spring bias to such an extent that the links will be grounded on the pivot stops for an extended period of time, i.e. so long as the load in the cab remains unevenly distributed. When this happens, vibrations from the guide rails and other sources are transmitted to the cab and passengers, and are not damped out by the link springs on the grounded guide roller links. The result is a lower quality bumpy ride in the elevator. The softer the springs, the better the ride, but more fly time is spent riding against the stoppers; hence, there is a tradeoff.
Japanese Kokai Publication No. 3-23185, published January 31, 1991, discloses a system for stabilizing an elevator cab as it is moving along guide rails in a hoistway, which guide rails possess a varying compliancy. The system includes transverse beams above and below the cab assembly which are adjustably movable relative to the cab assembly. Rail guides are mounted on the ends of the transverse beams by means of vibration-proof rubber pads. The beams are also connected to the cab assembly by vibration-proof rubber pads. A contoured guide piece is fixed to the hoistway wall which mimics the compliancy values of the rails, and contact sensors are mounted on the beams to slide over the guide piece. Motion of the contact sensors is monitored by a control which operates actuators operable to laterally shift the beams in response to movement of the contact sensors. The rail guide will thus be moved laterally relative to the cab assembly as the rail compliancy varies. A problem found in this Japanese teaching concerns the fact that if the beam is moved to the left to shift the left-hand rail guides in response to variations in compliancy of the left-hand rail, then the right-hand rail guides must necessarily move in the same direction as the left-hand rail guides. The objective of moving the rail guides toward a rail as rail compliancy increases, and away from the rail as rail compliancy decreases is thus only attainable on one of the rails, and the opposite rail guide movement occurs at the other opposite side rail. The use of the guide piece is also cumbersome, and its ability to mirror rail compliancy is problematic, at best.